In a machine tool such as a lathe, a processed work gripped by a spindle chuck is replaced with a new one by a loader or the like. In this case, rotation of a spindle is stopped, and the processed work is received by the loader. The loader then passes a new work to the spindle chuck. Thereafter, the spindle motor is rotated to actuate and accelerate the rotation of the spindle up to a speed required for processing. Thus, replacement of the work on the spindle chuck involves time required to decelerate and stop the rotation of the spindle, time required to deliver the work, and time required to properly rotate the spindle. The time required for work replacement increases the cycle time of processing. In particular, when processing with a tool in contact with the work requires only a short time, the time required for work replacement accounts for most of the cycle time.
A lathe enabling a reduction in the time required for work replacement has been proposed (for example, the Unexamined Japanese Patent Application Publication (Tokkai-Sho) No. 58-196904). In the lathe, two spindles are provided so as to be switchably placed in a processing position and in a standby position. While the spindle in the processing position is processing a work, the work on the spindle in the standby position is replaced with a new one. In this case, one spindle motor is used to drive the spindles and transmits power to the spindle in the processing position.
Another lathe has been proposed which has a fixed spindle and a plurality of spindle chucks that can be installed on and removed from the tip of the spindle. To attach any of the spindle chucks to the spindle during rotation, the lathe uses a dedicated motor that rotates the spindle chuck, and a spindle motor that rotationally drives the spindle. The lathe synchronously controls the rotation speeds of the dedicated motor and the spindle motor.
In the proposed example in which two spindles are rotated by the common spindle motor, the processing time can be utilized to receive the processed work from the spindle chuck and to deliver a new work to the spindle chuck. However, the acceleration of the rotation of the spindle up to the appropriate speed needs to be performed after the spindle in the standby position has reached the processing position. This prevents a reduction in the time required to accelerate the rotation of the spindle up to the appropriate speed and included in the work replacement time.
The lathe synchronously controlling the rotation speeds of the dedicated motor and the spindle motor needs to use two motors and to achieve the synchronous control during high-speed rotation. Furthermore, phase control is required to place the two motors in positions where the motors can be coupled together. Thus, the need for the plurality of motors increases the manufacturing costs of the machine tool as well as the size of the machine tool as a whole. Furthermore, the rotation speeds of the two motors need to be subjected to both synchronous control and phase control during high speed rotation. This complicates the control system and increases an arithmetic processing load on the control device.